Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 230937
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
337 AM CST Sun Nov 23 2014
.SHORT TERM: (Through Late This Afternoon)
Issued at 331 AM CST Sun Nov 23 2014
Surface low to begin lifting northeastward towards forecast area
today. Will see showers on the increase this morning and spread
north across the region. Some instability today so there is a chance
of thunderstorms mainly along and south of I-70. As for
temperatures, will depend on cloud cover and rainfall. But feel that
temps will rise into the mid 50s to low 60s by this afternoon.
.LONG TERM: (Tonight through Saturday)
Issued at 331 AM CST Sun Nov 23 2014
While specific precip trends are certainly tricky tonight and into
Monday, latest synoptic and hi-res guidance suggests the general
scenario will be that this afternoon`s band of rain and embedded
thunderstorms will rotate east out of the area this evening, as
additional precip begins to form to our west as secondary shortwave
drops south in the wake of today`s system. This secondary area of
precip should then wind down to spotty areas of light rain/light
snow during the day on Monday.
Yesterday`s guidance as well as 00z model output is now suggesting
more precip lingering in the cold air late tonight and early Monday
than was indicated by earlier runs. I have not made any major
changes to the timing of the changeover from rain to snow over the
northwest half of the CWA in the 06-12z time frame; this may be a
bit too fast, but didn`t want to discount the intensity of the
CAA. In the far NW counties, it appears that this strong CAA will
cause freezing levels to drop enough to support the rain changing
over to all snow for a few hours late tonight and into early
Monday morning, and based on this have added some minor accumulations
of under an inch in our extreme northwest counties.
Min/max temps for tonight/Monday will be based on non-diurnal, 3
hourly temp trends during this time frame.
No major changes in going forecast trends heading into midweek, with
dry and chlly weather on Tuesday followed by a brief warmup and low
PoPs for the fast-moving, moisture-starved system on Wednesday.
Medium range guidance has come into better agreement with a less
amplified solution for Thanksgiving Day, suggesting a ribbon of
light precip over northern sections of the CWA as shortwave
interacts with baroclinicity that is draped across the region.
Pattern suggests that there should be a strong temperature
gradient from north to south across the region, with highs in the
30s over the cold air in the north while approaching 50 in the
warmer air over southern sections of the CWA.
This agreement jumps the tracks heading into the latter half of the
week, with GEM and ECMWF forecasting broad trofing over the central
CONUS while the GFS maintains a more quasi-zonal look. Of course,
given the intensity of the baroclinic zone this somewhat subtle
difference will make a huge impact on sensible weather trends, and
especially on temperatures. This discrepency can be seen in the
surface prog forecasts for Friday morning; ECMWF and GEM have broad
and cold Canadian high over the plains and into the upper
Mississippi Valley with north/northeast winds over our FA, while GFS
as surface low over the northern plains and southerly winds over our
area. MOS guidance based on these solutions offer a more
quanitative and stark measure of this discrepency...a whopping
20-25 degree difference on Friday`s highs between the colder
ECMWF and warmer GFS. For now have maintained earlier (colder)
trends, based on consensus of GEM and ECMEF.
.AVIATION: (For the 06z TAFs through 06z Sunday Night)
Issued at 1136 PM CST Sat Nov 22 2014
Specifics for KCOU, KUIN: An approaching low pressure system will
bring widespread rainfall and IFR conditions to the area on
Sunday. Winds will become northwesterly behind a cold front
towards the end of the 24-hr TAF period at KUIN. Just beyond the
end of the TAF period, a brief period of light snow is possible
while precipitation is ending.
Specifics for KSTL, KSUS, KCPS: Despite several hours of
persistent light rainfall at night along with steadily increasing
surface dew points, ceilings have remained predominantly VFR at
the St. Louis metro area TAF sites over the last 6 hours. Most
perplexing. In any event, confidence in cig/vis trends is lower
during the first 0-6 hours of the TAF period compared to later,
when a strong low pressure system will bring widespread rainfall
and IFR conditions to the area. Conditions will deteriorate with
time, especially after 10-12z. Winds will become westerly to
northwesterly and increase behind a cold front towards the end of
the 30-hr TAF period.